standard reduction potential
Summary
Standard reduction potential (E°red) is the voltage of a reduction half-reaction under standard conditions (1 M, 1 atm, 25 °C), measured relative to the standard hydrogen electrode, defined as 0 V. The more positive it is, the stronger the oxidizing agent. The cathode is the half-cell with the higher E°red.
Detail
Meaning: Every half-reaction is written as a reduction (oxidized form + e⁻ → reduced form) and given a potential in volts, measured against the standard hydrogen electrode (2H⁺ + 2e⁻ → H₂), which is defined as 0 V. A more positive E°red means the species gains electrons more readily, so it is a stronger oxidizing agent. A more negative E°red means the reduced form is a stronger reducing agent.
Equation: E°cell = E°red(cathode) − E°red(anode), and ΔG° = −nFE°cell. E° is in volts (V). n is the moles of electrons transferred (mol). F is the Faraday constant, about 96,500 C/mol. ΔG° is in J/mol. A positive E°cell means ΔG° is negative, so the cell is spontaneous (galvanic).
On the MCAT: A passage gives a table of E°red values. You identify the cathode as the half-cell with the higher E°red, calculate E°cell, and predict spontaneity, the direction of electron flow, or which metal plates out and which dissolves. You may also rank oxidizing or reducing agents, or decide whether one species can reduce another.
Common confusion: Students multiply E° by the stoichiometric coefficients when doubling a half-reaction. Do not. E° is an intensive property, so only n changes, which affects ΔG°. Students also forget that reversing a half-reaction to show oxidation flips the sign of its potential, but the tabulated value is always for reduction. Do not subtract after flipping and also reverse the sign again.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.